The present invention relates to a device for measuring variations in the gain of a laser amplifier as a function of the pumping energy and to a process utilizing this device. It is used in optics, particularly in the production of laser amplifier chains.
Two processes are known for measuring the gain of a laser amplifier. The means for performing these processes are diagrammatically illustrated in FIGS. 1 and 2.
In the device shown in FIG. 1, it is desired to measure the gain of laser amplifier 10. It conventionally comprises an amplifier bar 12 around which is wound a flash tube 14 supplied with electric power by a circuit 16.
The device also comprises a laser oscillator 20 able to emit light radiation in the form of pulses 22 traversing the amplifier bar 12 at a distance R.sub.n from the axis of the bar. The wavelength of the radiation is that at which measurement of the gain is to take place. The pulse 22 is amplified by bar 12, when the latter is given amplifying characteristics under the action of the optical pumping produced by the flash tube. The output pulse 24 appearing at the amplifier output then has a higher energy level than that of the input pulse 22.
The system also comprises two semi-transparent plates 31, 32, placed respectively at the input and output of the amplifier bar 12 and two photosensitive detectors 33, 34 (calorimeters, photoelectric cells, etc) able to measure light energy.
A device of this type functions in the following manner. Plates 31 and 32 sample part of the input and output pulses 22, 24 respectively and transfer said part to detectors 33, 34, which measure the corresponding energy levels. The measurement of the ratio of these energies immediately gives the gain of bar 12 for the pumping energy value used and for the considered radius R.sub.n.
A number of measurements of this type for several values of E must be performed to obtain variations G(E) of the gain, as a function of the pumping energy E.
The second process is illustrated in FIG. 2, where the represented device also comprises a laser amplifier 10 whose gain is to be measured and a laser oscillator 40 which functions continuously, unlike the laser 20 of the preceding device. This laser emits radiation at the amplification wavelength of bar 12. When the latter is, for example, a neodynium-doped glass bar, laser 40 can, for example, be of the YAG type.
The represented device also comprises a beam splitter plate 42, a photoelectric detector 44 and a divider 46 of the signal supplied by detector 44 by a quantity proportional to the power supplied by laser 40.
The operation of this device is illustrated by means of the graph of FIG. 3 showing the variations in time of the signal supplied by detector 44. This signal represents the luminous power Ps(t) measured at the output of amplifier 10 for radius R.sub.n. When there is no optical pumping, this power is equal to the value Pe of the input radiation emitted by laser 40.
During pumping, the power Ps(t) increases up to a maximum value Pm, which is reached at a time tm and then decreases once again towards Pe. Thus, the gain of the amplifier bar, which is equal to the quotient Ps(t)/Pe varies as a function of time throughout the duration of the pumping pulse and passes through a maximum equal to Pm/Pe at time t=tm.
As in the case of the previously described device, when it is desired to obtain variations of the gain as a function of the pumping energy E, it is necessary to perform several measurements for different energy levels E and on each occasion sample the corresponding gain value, e.g. the maximum gain.
Compared with the previous process, this process has the advantage of permitting the determination of the time at which the gain is at a maximum, thereby making it possible to synchronize the various amplifiers of a laser chain.
The two prior art processes described hereinbefore have the disadvantage of requiring several pumping operations or "shots" for covering the complete desired energy range. This necessarily increases the measuring time. In addition, it reduces the life of the element to be measured.